I think you may have misunderstood what darwin is.
Darwin is the UNIX technology that Mac OS X is built upon. It is released under a public license so anyone can view the code. You can compile the source on a vatiety of platforms (including x86 - intel) but this wil only get you a basic command line operating system.
If you have mac os x then you already have darwin. I believe that 10.3.4 is built on darwin 7.4.0